In this study, we synthesized and characterized particles with a stearic acid (SA) core encapsulated in poly (methyl methacrylate) (PMMA) and four PMMA-hybrid shell materials. The objective was to create spherical particles with diameters of several hundred nanometers for use as the dispersed phase in thermotropic materials for overheat protection of polymer solar absorbers. Encapsulation was accomplished by emulsion polymerization. The composition, morphology and thermal properties of the encapsulated particles were characterized by scanning and transmission microscopy, Fourier transform infrared spectral analysis, digital scanning calorimetry and thermogravimetry. There were no chemical interactions between core and shell materials. The mean dia. meter and the thickness of the encapsulating shell varied over relatively narrow ranges of 110-360 nm, and 17-60 nm, respectively, depending on the functional groups of the shell. Based on published recommendations for size and shell thickness for thermotropic materials for overheat protection of polymer solar absorbers, particles encapsulated with polymethylmethacrylate-co-2-hydroxy-ethylacrylate (PMMA-HEA) shell and with polymethylmethacrylate-styrene (PMMA-S) shell are good candidates for further evaluation.